<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<script type="text/javascript">
			console.log(a); //undefined
			var a = 6;
			var a = 9;
			console.log(a);

			for (var i = 0; i < 3; i++) {
				console.log(i);
			}

			console.log(i);

			/*
			   1.声明前不能使用
			   2.不能重复声明
			   3.块级作用域
			*/
			// console.log(b);
			let b = 3;

			// let b = 5;
			for (let j = 0; j < 3; j++) {
				console.log(j);
			}
			console.log(j);
		</script>
	</head>
	<body>
	</body>
</html>
